Operation Barbarossa (German: Unternehmen Barbarossa) (Russian: ) was the invasion of the Soviet Union by Nazi Germany and most of its Axis allies, starting on Sunday, 22 June 1941, during World War II. The operation, code-named after Frederick Barbarossa ("red beard"), a 12th-century Holy Roman emperor and German king, put into action Nazi Germany's ideological goal of conquering the western Soviet Union to repopulate it with Germans. The German Generalplan Ost aimed to use some of the conquered people as forced labour for the Axis war effort while acquiring the oil reserves of the Caucasus as well as the agricultural resources of various Soviet territories. Their ultimate goal was to create more Lebensraum (living space) for Germany, and the eventual extermination of the indigenous Slavic peoples by mass deportation to Siberia, Germanisation, enslavement, and genocide.In the two years leading up to the invasion, Germany and the Soviet Union signed political and economic pacts for strategic purposes. Following the Soviet occupation of Bessarabia and Northern Bukovina, the German High Command began planning an invasion of the Soviet Union in July 1940 (under the codename Operation Otto). Over the course of the operation, over 3.8 million personnel of the Axis powersthe largest invasion force in the history of warfareinvaded the western Soviet Union along a 2,900-kilometer (1,800 mi) front, with 600,000 motor vehicles and over 600,000 horses for non-combat operations. The offensive marked a massive escalation of World War II, both geographically and in the formation of the Allied coalition including the Soviet Union.

The operation opened up the Eastern Front, in which more forces were committed than in any other theater of war in history. The area saw some of the world's largest battles, most horrific atrocities, and highest casualties (for Soviet and Axis forces alike), all of which influenced the course of World War II and the subsequent history of the 20th century. The German armies eventually captured some five million Soviet Red Army troops. The Nazis deliberately starved to death or otherwise killed 3.3 million Soviet prisoners of war, and millions of civilians, as the "Hunger Plan" worked to solve German food shortages and exterminate the Slavic population through starvation. Mass shootings and gassing operations, carried out by the Nazis or willing collaborators, murdered over a million Soviet Jews as part of the Holocaust.The failure of Operation Barbarossa reversed the fortunes of Nazi Germany. Operationally, German forces achieved significant victories and occupied some of the most important economic areas of the Soviet Union (mainly in Ukraine) and inflicted, as well as sustained, heavy casualties. Despite these early successes, the German offensive stalled in the Battle of Moscow at the end of 1941, and the subsequent Soviet winter counteroffensive pushed the Germans about 250 km (160 mi) back. The Germans had confidently expected a quick collapse of Soviet resistance as in Poland, but the Red Army absorbed the German Wehrmacht's strongest blows and bogged it down in a war of attrition for which the Germans were unprepared. The Wehrmacht's diminished forces could no longer attack along the entire Eastern Front, and subsequent operations to retake the initiative and drive deep into Soviet territorysuch as Case Blue in 1942 and Operation Citadel in 1943eventually failed, which resulted in the Wehrmacht's retreat and collapse.

Nazi Germany, officially known as the German Reich from 1933 until 1943, and the Greater German Reich from 1943 to 1945, was the German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a dictatorship. Under Hitler's rule, Germany quickly became a totalitarian state where nearly all aspects of life were controlled by the government. The Third Reich, meaning "Third Realm" or "Third Empire", alluded to the Nazi claim that Nazi Germany was the successor to the earlier Holy Roman Empire (800–1806) and German Empire (1871–1918). The Third Reich, which Hitler and the Nazis referred to as the Thousand Year Reich, ended in May 1945 after just 12 years, when the Allies defeated Germany, ending World War II in Europe.

On 30 January 1933, Hitler was appointed chancellor of Germany, the head of government, by the president of the Weimar Republic, Paul von Hindenburg, the head of state. The Nazi Party then began to eliminate all political opposition and consolidate its power. Hindenburg died on 2 August 1934 and Hitler became dictator of Germany by merging the offices and powers of the chancellery and presidency. A national referendum held 19 August 1934 confirmed Hitler as sole Führer (leader) of Germany. All power was centralised in Hitler's person and his word became the highest law. The government was not a coordinated, co-operating body, but a collection of factions struggling for power and Hitler's favour. In the midst of the Great Depression, the Nazis restored economic stability and ended mass unemployment using heavy military spending and a mixed economy. Using deficit spending, the regime undertook a massive secret rearmament program, forming the Wehrmacht (armed forces), and constructed extensive public works projects, including the Autobahnen (motorways). The return to economic stability boosted the regime's popularity.

Racism, Nazi eugenics, and especially antisemitism, were central ideological features of the regime. The Germanic peoples were considered by the Nazis to be the master race, the purest branch of the Aryan race. Discrimination and the persecution of Jews and Romani people began in earnest after the seizure of power. The first concentration camps were established in March 1933. Jews and others deemed undesirable were imprisoned, and liberals, socialists, and communists were murdered, imprisoned, or exiled. Christian churches and citizens that opposed Hitler's rule were oppressed and many leaders imprisoned. Education focused on racial biology, population policy, and fitness for military service. Career and educational opportunities for women were curtailed. Recreation and tourism were organised via the Strength Through Joy program, and the 1936 Summer Olympics showcased Germany on the international stage. Propaganda Minister Joseph Goebbels made effective use of film, mass rallies, and Hitler's hypnotic oratory to influence public opinion. The government controlled artistic expression, promoting specific art forms and banning or discouraging others.

From the latter half of the 1930s, Nazi Germany made increasingly aggressive territorial demands, threatening war if these were not met. The Saarland voted by plebiscite to rejoin Germany in 1935, and in 1936 Hitler sent troops into the Rhineland, which had been de-militarized after World War I. Germany seized Austria in the Anschluss of 1938, and demanded and received the Sudetenland region of Czechoslovakia in that same year. In March 1939, the Slovak state was proclaimed and became a client state of Germany, and the German Protectorate of Bohemia and Moravia was established on the remainder of the occupied Czech Lands. Shortly after, Germany pressured Lithuania into ceding the Memel Territory. Germany signed a non-aggression pact with the Soviet Union and invaded Poland on 1 September 1939, launching World War II in Europe. By early 1941, Germany and its European allies in the Axis powers controlled much of Europe. Extended offices of the Reichskommissariat took control of Nazi-conquered areas and a German administration was established in the remainder of Poland. Germany exploited the raw materials and labour of both its occupied territories and its allies.

Genocide, mass murder, and large-scale forced labour became hallmarks of the regime. Starting in 1939, hundreds of thousands of German citizens with mental or physical disabilities were murdered in hospitals and asylums. Einsatzgruppen paramilitary death squads accompanied the German armed forces inside the occupied territories and conducted the genocide of millions of Jews and other Holocaust victims. After 1941, millions of others were imprisoned, worked to death, or murdered in Nazi concentration camps and extermination camps. This genocide is known as the Holocaust.

While the German invasion of the Soviet Union in 1941 was initially successful, the Soviet resurgence and entry of the United States into the war meant that the Wehrmacht lost the initiative on the Eastern Front in 1943 and by late 1944 had been pushed back to the pre-1939 border. Large-scale aerial bombing of Germany escalated in 1944 and the Axis powers were driven back in Eastern and Southern Europe. After the Allied invasion of France, Germany was conquered by the Soviet Union from the east and the other Allies from the west, and capitulated in May 1945. Hitler's refusal to admit defeat led to massive destruction of German infrastructure and additional war-related deaths in the closing months of the war. The victorious Allies initiated a policy of denazification and put many of the surviving Nazi leadership on trial for war crimes at the Nuremberg trials.